The present invention relates to a traffic shaper in an asynchronous-transfer-mode communication network, more particularly to a traffic shaper that controls the flow of cells into a virtual-path-switching network.
The asynchronous transfer mode (hereinafter, ATM) is advantageous for multimedia communication and is currently employed in many local area networks. An ATM network transports fixed-length packets, referred to as cells, on a plurality of virtual paths, each comprising one or more virtual channels. A virtual-path-switching network switches the cell traffic on a virtual-path basis by an asynchronous time-division multiplexing scheme in which the virtual paths do not have fixed time-slot assignments.
Standards for the use of ATM in broadband integrated-services digital networks (B-ISDNs), and for traffic control and congestion control in such networks, have been recommended by the Telecommunication Standardization Sector of the International Telecommunication Union, e.g. in ITU-T Recommendation I.371. Traffic management specifications have also been drafted by the Technical Committee of the ATM Forum. These standards and specifications indicate methods of regulating the rate of cell flow on virtual paths and channels within an ATM network.
The function of a traffic shaper is to control the cell flow into the network so that the traffic on each virtual path is kept within the rate allowed for the path. A conventional traffic shaper maintains a single first-in-first-out queue for each virtual path being controlled. Arriving cells are placed in the queue specified by address information in the cell header. Cells are taken from each queue at a rate not exceeding a specified peak cell rate.
The peak cell rate is one of the several parameters determining the class of service provided on the virtual path. Other parameters concern such quality factors as cell loss ratio, cell transfer delay, and the variation in this delay. To meet the requirements of different types of cell traffic, a network may have to provide many classes of service. With the conventional traffic shaper, since all cells placed in the same queue receive basically the same class of service, a large number of separate virtual paths are required, placing a strain on the network's virtual-path-switching resources.